Mentioning the need for racer flag in Rust section

This covers users that have already installed YCM before and are looking
to set up Rust completion. They're unlikely to start reading from the
Installation section, so we mention the need for the new build flag to
prevent "why doesn't it work for me" confusion.

README.md

@@ -798,7 +798,7 @@ your file.
 
 #### Quick start
 
-1. Ensure that you have enabled the Tern completer. See the 
+1. Ensure that you have enabled the Tern completer. See the
    [installation guide](#installation) for details.
 
 2. Create a `.tern-project` file in the root directory of your JavaScript
@@ -890,11 +890,13 @@ define( [ 'mylib/file1', 'anotherlib/anotherfile' ], function( f1, f2 ) {
 
 ### Rust semantic completion
 
-Completions and GoTo* within the current crate and its dependencies should work
-out of the box with no additional configuration. For semantic analysis inclusive
-of the standard library, you must have a local copy of
-[the rust source code][rust-src]. You also need to set the following option so
-YouCompleteMe can locate it.
+Completions and GoTo commands within the current crate and its dependencies
+should work out of the box with no additional configuration (provided that you
+built YCM with the `--racer-completer` flag; see the [*Installation*
+section](#installation) for details).  For semantic analysis inclusive of the
+standard library, you must have a local copy of [the rust source
+code][rust-src]. You also need to set the following option so YouCompleteMe can
+locate it.
 
 ```viml
 " In this example, the rust source code zip has been extracted to